The primary cell walls of graminaceous monocots were known to have a low content of pectin compared to those of dicots, but it was uncertain how widespread this feature was within the monocots as a whole. Nonlignified cell walls were therefore prepared from 33 monocot species for determination of their pectin content. It was not possible to solubilize intact pectins quantitatively from the cell walls, and the pectin content was assessed from three criteria: the total uronic acid content; the content of a-(1,4')-D-galacturonan isolated by partial hydrolysis and characterized by electrophoresis and degradation by purified polygalacturonase; and the proportion of neutral residues in a representative pectic fraction solubilized by sequential fl-elimination and N,N,N'N'-cyclohexanediaminetetraacetic acid extraction. Low galacturonan contents were restricted to species from the Gramineae, Cyperaceae, Juncaceae, and Restionaceae. Other species related to these had intermediate galacturonan contents, and the remainder of the monocots examined had high galacturonan contents comparable with those of dicots. The other criteria of pectin content showed the same pattern.The primary cell walls of the Gramineae (Poaceae) are quite different from those of dicots and gymnosperms. They contain little pectic material, and their main hemicelluloses are arabinoxylans (21). In fact, their composition is more like that of the secondary cell walls of woody dicots, although with little or no lignin and with differences in the detailed structure and chain packing of the polysaccharides.Formerly, all monocots were presumed to have primary cell walls like those of the Gramineae, few other species having been examined. There is now a growing realization that some species in the Liliflorae and certain other monocot groups contain pectic polysaccharides in the quantities expected of dicots (10,13,18,24,26